Inductive noise and capacitive coupling shielding

How inductive noise is introduced

Inductive (magnetic) coupling occurs when a changing current in one circuit produces a changing magnetic flux that links with another loop. A noise voltage is induced in the victim loop:

V_noise = M · dI/dt (M = mutual inductance, in henry)

M increases with larger loop areas, long parallel conductors and closer spacing. The induced voltage also increases with frequency, because a higher frequency means a larger dI/dt. Sources are motors, relays, transformers, switching supplies and power cables carrying high di/dt. The induced voltage appears in series with the victim loop and is independent of the loop's impedance.

 Loop 1 (noisy)      Loop 2 (victim)
   ___ I(t)           ___
  |   |   <-- flux --> |   |  V = M dI/dt
  |___|                |___|

Shielding for capacitive coupling

Capacitive coupling passes a current I = C·dV/dt through a stray capacitance C between the source and the receiver. An electrostatic (Faraday) shield works as follows:

1. Place a conductive shield (metal foil, braid or enclosure) around the receiver or between the two circuits.
2. Connect the shield to ground (the reference of the circuit) at a proper point. The stray capacitance is split into source-to-shield and shield-to-receiver capacitances.
3. The noise current goes from the source to the shield, then to ground, and does not reach the receiver. The remaining coupling capacitance (source to receiver) becomes very small.

 Source --Cs1--[ SHIELD ]--Cs2-- Receiver
                  |
                 GND

An ungrounded or badly grounded shield does not help (it may even increase the coupling). Keep the shield leads short and cover the receiver conductor completely.